1. Technical Field
The present invention relates to a method and apparatus for reading data in probe-based data storage devices.
2. Discussion of Related Art
Techniques employing nanometer-sharp tips for imaging and investigating the structure of materials down to the atomic scale, such as the atomic force microscope (AFM) and the scanning tunneling microscope (STM), also find application in ultra high density storage devices. See, for example, U.S. Pat. No. 4,575,822 and P. Vettiger et al., “The Millipede—More than one thousand tips for future AFM data storage,” IBM Journal of Research and Development, vol. 44 No. 3 May 2000, pp. 323–340. In the device described by Vettiger, information is stored as sequences of “pits” and “no pits” written on a polymer storage surface via an array of cantilevers each carrying a tip. The cantilevers, and the tips thereon, are selectively heated to write data onto the surface. The heating of each tip to a sufficient level produces a corresponding deformation of the surface adjacent the tip. The stored information is read back by treating each cantilever as a thermo-mechanical sensor in a circuit which is electrically equivalent, to a first degree of approximation, to a current source or a voltage source in cascade with a variable resistor. The sensor transforms the physical value carrying the read information into an electrical signal. The value of the variable resistor depends on the temperature at the tip of the cantilever. During the read process, the cantilever reaches different temperatures whether it moves over a “pit” (bit “1”) or a “no pit” (bit “0”). A detection circuit senses a voltage which is dependent on the value of the cantilever resistance to make a decision on whether a “1” or a “0” is detected.
Conventionally, to read the recorded information, the cantilever employed for writing is provided with the additional function of a thermal read back sensor by exploiting its temperature dependent resistance. In general, the resistance increases non-linearly with heating power/temperature from room temperature to a peak value of 500–700 degrees C. The peak temperature is determined by doping concentration in the variable resistance of the cantilever, which ranges from 1×1017 to 2×1018 cm−3. Above the peak temperature, the resistance drops as the number of intrinsic carriers increases through thermal excitation. For sensing, the resistor is operated at about 350° C. This temperature is not high enough to deform the surface as in the case of writing.
The principle of thermal sensing is based on the thermal conductance between the heater platform and the surface changing according to the distance between them. The medium between the heater platform and the surface, such as air, transports heat from the cantilever to the surface. When the distance between cantilever and surface is reduced as the tip moves into a pit, the heat transport through the air becomes more efficient. As a result, the evolution of the heater temperature in response to a pulse applied to the cantilever is different and, in particular, the maximum value achieved by the temperature is smaller than in the case in which no pit is present. As the value of the variable resistance depends on the temperature of the cantilever, the maximum value achieved by the resistance will be smaller as the cantilever moves over a pit. Therefore, during the read process, the cantilever resistance reaches different values whether it moves over a pit (bit “1”) or no pit (bit “0”).
The thermo-mechanical cantilever sensor, which transforms temperature into an electrical signal that carries information, is electrically equivalent, to a first degree of approximation, to a variable resistance. A detection circuit should therefore sense a voltage that depends on the value of the cantilever resistance to make a decision on whether a “1” or a “0” is written. The relative variation of thermal resistance is typically around 10−5/nm. Hence, a written bit “1” typically produces a relative change of the cantilever thermal resistance ΔRΘ/RΘ of about 10−4˜5×10−4. The relative change of the cantilever electrical resistance is of the same order of magnitude. As a consequence, an important issue in detecting the presence or absence of a pit is a sufficiently high resolution to permit extraction of the signal that contains the information about the bit being “1” or “0”. The signal carrying the information can be viewed as a small signal superimposed to a very large offset signal, which can be three to four orders of magnitude larger.
Parallel operation of large two-dimensional arrays can be achieved by a row/column time-multiplexed addressing scheme similar to that implemented in DRAMs. In the device described in [5], such a multiplexing scheme is employed to address the array column by column for parallel write/read operation within one column. In particular, read back signal samples are obtained by applying a read pulse to the cantilevers in a column of the array, low-pass filtering the cantilever response signals, and sampling the filter output signals. This process is repeated sequentially until all columns of the array are addressed, and then restarted from the first column. The time between two pulses corresponds to the time needed for a cantilever to move from one bit position to the next. Another problem encountered with time-multiplexed read operations based on thermo-mechanical sensing stems from an inherent limitation to achievable data rate which is determined by the cantilever thermal time constant. Specifically, a read pulse needs a duration at least equal to the time taken for the cantilever to achieve a temperature of about 350 degrees C., at which reading can take place.